Transmission apparatus, transmission method, communication system, and method of same

ABSTRACT

To provide a transmission apparatus and a communication system that transmit a plurality of transmission signals riding on different interfering transmission beams, separate and receive the transmission signal at the receiving side, and estimate the original transmission signal by maximum likelihood estimation based on the correlation among the received signals. A transmission apparatus which encodes the transmission signal by an encoding means to generate at least two transmission signals S 1  and S 2 , modulates the signals to the carrier frequencies, weights them and supplies them to the antenna elements, and controls the weights to transmit the modulated output signals by beams partially overlapping each other in space. A reception apparatus which separates and receives the transmission signals by a channel adaptive antenna to receive at least two received signals RS 1  and RS 2  and estimates the original transmission signal using maximum likelihood estimation, for example, a Viterbi decoding algorithm, based on the correlation between the received signals so as to reduce the error rate of the received signal and realize an improvement of the quality of communication.

TECHNICAL FIELD

The present invention relates to a transmission apparatus performingspace-time analog coding for transmission signals using an antenna, acommunication system using the transmission apparatus, and atransmission method and communication method of the same.

BACKGROUND ART

In recent years, in mobile communication, there has been a demand forefficient communication in both space and time from the viewpoint ofdealing with different communication capacities between uplinks anddownlinks, that is, so-called non-symmetrical channels, thesimplification of hardware at the mobile station side, and theimprovement of the efficiency of utilization of the frequency.Particularly, in view of the adoption of W-CDMA (wide band code divisionmultiple access) as a wireless access scheme for the next generationmobile communication system IMT2000, transmission diversity using aplurality of antenna elements, adaptive antenna arrays using a pluralityof antenna elements, and other systems have been proposed as powerfultechnologies for increasing the capacities of the W-CDMA system.

However, in the prior art described above, there has been almost nostudy relating to the correlation of the transmission signals due to theinterference among beams in the case of transmission by a plurality oftransmission beam at the transmitting side. Further, the correlation ofthe transmission signals due to the interference among beams has notbeen utilized actively to improve the communication efficiency and toimprove the quality of communication. Thus, in the case of transmittingand receiving signals by forming a plurality of beams, that is,so-called space diversity, the interference among beams has almostalways been suppressed as much as possible to suppress interference andthe correlation of transmission signals has not been utilizedeffectively.

DISCLOSURE OF THE INVENTION

The present invention was made in consideration of the abovecircumstances and has as an object to provide a transmission apparatusand a communication system capable of, when transmitting signals byforming a plurality of transmission beams, giving correlation to thetransmission signals according to the overlap among beams and estimatingthe original transmission signal by maximum likelihood estimationconsidering the correlation among the plurality of received signalsseparated and received at the receiving side so as to reduce the errorrate of the received signals and realize an improvement of thetransmission efficiency and improvement of the quality of communication.

To achieve the above object, according to the present invention, thereis provided a transmission apparatus comprising an encoding means forencoding a transmission signal to generate at least a first transmissionsignal and a second transmission signal, a transmission antenna formingat least a first beam and a second beam, and a transmitting means fortransmitting said first transmission signal riding on said first beamand transmitting said second transmission signal riding on said secondbeam, wherein said first beam and second beam are formed so as topartially overlap each other.

Further, in the present invention, preferably the transmission antennais an array antenna comprising a plurality of antenna elements, and thetransmitting means weights the first transmission signal and the secondtransmission signal modulated to predetermined carrier frequencies withpredetermined weights and supplies them to the antenna elements.

Further, in the present invention, the apparatus further comprises aweight determining means for determining weights of the antenna elementsand controlling the beam patterns of the first beam and the second beam,and the weight determining means determines the weights of the antennaelements in accordance with the channel characteristics.

Further, the communication system of the present invention comprises anencoding means for encoding a transmission signal to generate at least afirst transmission signal and a second transmission signal, atransmission antenna forming at least a first transmission beam and asecond transmission beam, a transmitting means for transmitting saidfirst transmission signal riding on said first transmission beam andtransmitting said second transmission signal riding on said second beam,a reception antenna forming a predetermined receiving beam and receivingsignals transmitted by said transmission antenna using the receivingbeam, and a decoding means for estimating the transmission signal by amaximum likelihood estimation according to the received signals of thereception antenna, wherein said first transmission beam and secondtransmission beam are formed so as to partially overlap each other.

Further, in the present invention, the transmission antenna is an arrayantenna comprising a plurality of antenna elements.

Further, in the present invention, the reception antenna is an arrayantenna comprising a plurality of antenna elements, and the receptionantenna forms a first receiving beam and a second receiving beam andseparates and receives the signals transmitted by the transmissionantenna by the first receiving beam and the second receiving beam.

Further, in the present invention, the decoding means estimates theoriginal transmission signal based on the correlation between thereceived signal received by the first receiving beam and the receivedsignal received by the second receiving beam for example by a Viterbidecoding algorithm.

Further, the communication method of the present invention comprises astep of encoding a transmission signal to generate at least a firsttransmission signal and a second transmission signal, a step ofmodulating the first transmission signal and the second transmissionsignal to predetermined carrier frequencies, and a step of weighting thefirst transmission signal by a first weight, supplying the same toantenna elements constituting a transmission antenna, and transmittingthe same by forming a first transmission beam and of weighting thesecond transmission signal by a second weight, supplying the same toantenna elements, and transmitting the same by forming a secondtransmission beam so as to partially overlap the first transmissionbeam.

Further, the communication method of the present invention comprises astep of encoding a transmission signal to generate at least a firsttransmission signal and a second transmission signal, a step ofmodulating the first transmission signal and the second transmissionsignal to predetermined carrier frequencies, a step of weighting thefirst transmission signal by a first weight, supplying the same toantenna elements constituting a transmission antenna, and transmittingthe same by forming a first transmission beam and of weighting thesecond transmission signal by a second weight, supplying the same toantenna elements, and transmitting the same by forming a secondtransmission beam so as to partially overlap the first transmissionbeam, a step of shaping a beam for compensation of the transmissiondistortion of the channel and receiving the signals sent by thetransmission antenna by the shaped beam, and a step of estimating theoriginal transmission signal in accordance with the received signals bythe reception antenna by maximum likelihood estimation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an embodiment of a communication systemaccording to the present invention.

FIG. 2 is a view of an example of the configuration of a transmissionapparatus in the communication system of the present invention.

FIG. 3 is a graph of an example of an antenna beam pattern.

FIG. 4 is a circuit diagram of an example of a beam control circuit of atransmission apparatus.

FIG. 5 is a view of the concept of a mathematic model of a transmissionapparatus.

FIG. 6 is a view of another example of a transmission apparatus of thepresent invention, wherein the encoder is comprised of a delay circuit.

FIG. 7 is a view of an example of the configuration of a transmissionapparatus including a weight vector determination circuit for finding aweight function.

FIG. 8 is a view of an example of the configuration of a receptionapparatus.

FIG. 9 is a view of a trellis chart of Viterbi decoding in a decodingcircuit of the reception apparatus.

FIG. 10 is a graph for comparison of the bit error rate of thecommunication scheme of the present invention with that of the diversitytransmission scheme of the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a view of the configuration of an embodiment of acommunication system according to the present invention. As illustrated,the communication system of the present embodiment is comprised of atransmission apparatus 100, a channel 200, and a reception apparatus300.

The transmission apparatus 100 encodes the transmission signal Si,modulates it to a carrier frequency, then transmits it by an antennaarray comprised of a plurality of antenna elements. The signaltransmitted by the transmission antenna is transmitted to the receptionapparatus 300 through the channel 200.

The channel 200 is a transmission path between the transmissionapparatus 100 and the reception apparatus 300. Generally, thetransmission characteristics of the channel 200 are not constant andchange along with time. Particularly, in a mobile communication system,for example, when the transmission apparatus 100 is a base station, thereception apparatus 300 is a mobile communication terminal, and thereception apparatus 300 commutes while moving, the transmissioncharacteristics and the distortion characteristics of the channel 200change all the time.

The reception apparatus 300 demodulates the signal received by thereception antenna, decodes it by decoding processing, and outputs adecoded signal S₀ closest to the original transmission signal.Generally, the decoded signal S₀ does not completely replicate theoriginal transmission signal due to the distortion of the channel 200.Error occurs at a certain error rate. The error rate of the decodedsignal S₀ is reduced by the encoding processing performed in thetransmission apparatus 100 and the decoding processing performed in thereception apparatus 300.

As shown in FIG. 1, in the transmission apparatus 100 of the presentembodiment, a plurality of transmission beams are formed by an antennaarray comprising a plurality of antenna elements (in FIG. 1, two beamsB1 and B2 are displayed as an example). The beams B1 and B2 overlap eachother in part of the beam patterns in space. That is, the signalstransmitted by the beams B1 and B2 interfere with each other inaccordance with the beam patterns of the antenna array and arecorrelated with each other.

FIG. 2 shows an example of the configuration of the transmissionapparatus 100. As illustrated, the transmission apparatus 100 iscomprised of an encoder 110, a modulator 120, a beam control circuit130, and an antenna array 140. Below, the configuration and function ofeach part of the transmission apparatus 100 will be explained in detailwith reference to FIG. 2.

The encoder 110 encodes an input signal S_(i) by a predeterminedencoding scheme and outputs at least two different strings of data (alsoreferred to as “bit streams”) D1 and D2. Note that, in the presentembodiment, the encoding scheme in the encoder 110 and the number of thedata strings output are not particularly limited. The optimum encodingscheme is selected in accordance with need. Further, the bit rates ofthe output data strings differ in accordance with the encoding schemeselected. For example, the encoder 110 generates a data string D1 with acoding rate of ½ by convolutional coding in accordance with the inputsignal S_(i) and generates a data string D2 with a coding rate of ⅓ byanother convolutional coding in accordance with the input signal S_(i)in parallel with the generation of the data string D1. Therefore, thebit rates r1 and r2 of the two data strings D1 and D2 output by theencoder 110 are different, in this example, r1/r2=⅔.

The data strings D1 and D2 output by the encoder 110 are modulated tocarrier frequencies by the modulator 120, respectively. Note that themodulator 120, for example, can spread the frequency spectrum of thetransmission signal using a spread sequence such as in the CDMAcommunication scheme.

As a result of frequency modulation, two transmission signals S1 and S2modulated to the carrier frequencies are generated and supplied to thebeam control circuit 130.

The beam control circuit 130 controls the weights of the antennaelements so as to control the beam pattern of the antenna array 140 inaccordance with the weights of the antenna elements. The beam controlcircuit 130 controls the beam pattern of the antenna array 140 bycontrolling the amplitudes and phases of transmission signals suppliedto the antenna elements in accordance with their given weight functions.

The antenna array 140 is comprised of a plurality of antenna elements.Since the beam control circuit 130 supplies weighted transmissionsignals to the antenna elements, the beam pattern at the time oftransmission is controlled in accordance with the weights of the antennaelements. The weights of the signals supplied to the antenna elementsare controlled according to weight functions or weight vectors.

FIG. 3 is a graph showing an example of a transmission beam pattern ofthe antenna array. As illustrated, the antenna pattern is formed withtwo beams B1 and B2. Here, assuming that the peak directions of the mainlobes of the beams B1 and B2 are θ1 and θ2, respectively, in the exampleof FIG. 3, θ1=0, and θ2=10 degrees.

To form the beam patterns B1 and B2 shown in FIG. 3, it is sufficient tosupply the antenna elements with transmission signals controlled inamplitude and phase in accordance with the weight vectors W1 and W2.Here, if the number of the antenna elements constituting the antennaarray 130 is n, the weight vectors W1 and W2 can be represented asvectors of the power n as shown in the following equations:W1=[1, e ^(−jπsin θ) ¹ , e ^(−j2πsin θ) ¹ , . . . , e ^(−j(n−1)πsin θ) ¹]  (1)W2=[1, e ^(−jπsin θ) ² , e ^(−j2πsin θ) ² , . . . , e ^(−j(n−1)πsin θ) ²]  (2)

The beam control circuit 130 weights the transmission signals inaccordance with the weight vectors and supplies the same to the antennaelements.

FIG. 4 is a circuit diagram of an example of the configuration of thebeam control circuit 130. In this example, the beam control circuit 130controls the amplitudes and phases of the two transmission signals S1and S2 output from the modulator 120 in accordance with the two weightvectors W1 and W2 and sends the signals to the antenna elements.

As shown in FIG. 4, the beam control circuit 130 is comprised of complexmultipliers 131-1, 131-2, . . . , 131-n, 132-1, 132-2, . . . , 132-n andadders 133-1, 133-2, . . . , 133-n.

Here, the weight vector W1 is represented as [w1(1), w1(2), . . . ,w1(n)] and the weight vector W2 is represented as [w2(1), w2(2), . . . ,w2(n)].

The modulated output signal S1 is multiplied by the n number of elementsw1(1), w1(2), . . . , w1(n) of the weight vector W1 by complexmultipliers 131-1, 131-2, . . . , 131-n, respectively. Similarly, themodulated signal S2 is multiplied by the n number of elements w2(1),w2(2), . . . , w2(n) of the weight vector W2 by the complex multipliers132-1, 132-2, . . . , 132-n, respectively.

The outputs of the multipliers 131-1 and 132-1 are added by the adder133-1 and supplied to the antenna element 141-1, the outputs of themultipliers 131-2 and 132-2 are added by the adder 133-2 and supplied tothe antenna element 141-2, . . . , and the outputs of the multipliers131-n and 132-n are added by the adder 133-n and supplied to the antennaelement 141-n.

The antenna elements 141-1, 141-2, . . . , 141-n generate radio waves inaccordance with the signals supplied by the adders 133-1, 133-2, . . . ,133-n and radiate the same to space. As a result, the two transmissionbeams B1 and B2 are generated by the antenna array 140, and the twomodulated signals S1 and S2 output by the modulator 120 are transmittedriding on different beams B1 and B2, respectively.

The beams B1 and B2, for example, have the patterns shown in FIG. 3.Namely, the beams B1 and B2 overlap each other partially in the spacedomain. Consequently, the transmission signals S1 and S2 sent by thebeams B1 and B2 interfere with each other and have correlation. Here,the interferences between the beams B1 and B2 are made α and β,respectively. That is, the amount of the interference to the beam B1 bythe beam B2 is assumed to be α, and the amount of the interference tothe beam B2 by the beam B1 is assumed to be β.

Here, by assuming that steering vectors for forming the beams B1 and B2are A1(=[a1(1), a1(2), . . . , a1(n)], and A2(=[a2(1), a2(2), . . . ,a2(n)], the amounts of the interference α and β can be given by thefollowing equations:

$\begin{matrix}{\alpha = \frac{{W2} \times {A1}^{T}}{{W1} \times {A1}^{T}}} & (3) \\{\beta = \frac{{W1} \times {A2}^{T}}{{W2} \times {A2}^{T}}} & (4)\end{matrix}$

In Equations (3) and (4), A1 ^(T) and A2 ^(T) are transposed matrixes ofthe steering vectors A1 and A2, respectively.

The mathematic model of the transmission apparatus of the presentembodiment in accordance with the amounts of the interference α and βbetween the beams B1 and B2 can be shown in FIG. 5. As illustrated, thetransmitted signal S_(i) is encoded by the encoder 110 and modulated,then two modulated output signals S1 and S2 are output. Since thesemodulated output signals are transmitted riding on the beams B1 and B2of the antenna array 140, respectively, the signal transmitted riding onthe beam B2 is mixed with the signal transmitted by the beam B2.Similarly, the signal transmitted by the beam B1 is mixed with thesignal transmitted by the beam B1.

Here, the two modulated output signals output-by the modulator 120 aremade S1(t) and S2(t), and the modulated output signals S1(t) and S2(t)are input to the beam control circuit 130, respectively. The beamcontrol circuit 130 supplies the transmission signals weighted by theweight vectors to the antenna elements and emits them to space by thetransmission beams B1 and B2 formed by the antenna elements. As a resultof the interference between the beams B1 and B2, the emitted signalsX1(t) and X2(t) in the directions of the main lobes of the beams B1 andB2 can be represented as in the following equations.X1(t)=S1(t)+αS2(t)  (5)X2(t)=S2(t)+βS1(t)  (6)

In this way, the two signals S1(t) and S2(t) transmitted by the beams B1and B2 are emitted into space while interfering with each other. Thus,the receiving side, for example, separates and receives the transmittedsignals by a reception antenna formed with two or more receiving beams,As a result, there is correlation between the at least two receivedsignals. In accordance with the correlation of the received signals, theerror rates of the decoded signals can be reduced and the reliability ofthe communication system improved by maximum likelihood estimation, forexample, Viterbi decoding.

FIG. 6 shows another example of the configuration of the transmissionapparatus of the present embodiment. As illustrated, in the transmissionapparatus 100 a, except for the difference of the encoder 110 a from theencoder 110 of the transmission apparatus shown in FIG. 2, the rest ofthe configuration is substantially the same as the transmissionapparatus 100 shown in FIG. 2.

The transmission apparatus 100 a is comprised of a delay circuit 112.The input signal S_(i) is, for example, a data string generated based onpredetermined information source data. The input signal S_(i) is, on theone hand, input to the modulator 120 as a data string D1, and on theother hand, input to the delay circuit 112. The delay circuit 112 delaysthe bits of the input data string by a predetermined delay time,generates a data string D2 that is delayed in time compared to the inputsignal Si, then inputs it to the modulator 120.

As described above, in the transmission apparatus 100 a, the encoder 110asupplies the delayed signal of the input signal S_(i) along with theoriginal signal to the modulator 120 as an encoded data string.

The partial circuits following the encoder 110 a have substantially thesame configuration and function in the same way as the portions of thetransmission apparatus 100 shown in FIG. 2. Namely, the modulator 120generates the modulated signals S1 and S2 modulated to predeterminedcarrier frequencies in accordance with the two input data strings D1 andD2 and outputs them to the beam control circuit 130. The beam controlcircuit 130 generates the signals supplied to the antenna elements ofthe antenna array 140 in accordance with predetermined weight vectors.In the antenna array 140, the radio waves corresponding to the signalssupplied by the antenna elements are emitted to space, so the radio wavesignals are emitted by the beam patterns set by the weight vectors, andthe radio wave signals are transmitted by two transmission beamspartially overlapping each other in space.

That is, by utilizing the encoder 100 a of this example, using the delaycircuit 112 for delaying the input signal by a predetermined delay time,the original information data strings are sent twice shifted in time byexactly the delay time of the delay circuit 112. For example, bytransmitting the original signal and the delayed signal thereof ridingon different beams, when the reception apparatus separates and receivesthe signals transmitted by the two beams, the two received signals havecorrelation with each other. In the reception apparatus, by decoding thereceived signals by maximum likelihood estimation, for example, Viterbidecoding, the error rates of the signals reproduced by the decoding aregreatly reduced, so the reliability of the communication system can beimproved.

FIG. 7 shows another example of the configuration of the transmissionapparatus. As illustrated, in this example, a weight vectordetermination circuit 150 for providing the weight vectors W1 and W2 tothe beam control circuit 130 is provided in the transmission apparatus100 b.

As described above, in the beam control circuit 130, by supplyingsignals changed in amplitudes and phases of the modulated output signalsS1 and S2 in accordance with the given weight vectors to the antennaelements of the antenna array 140, the predetermined beams are formed.For example, two beams partially overlapping in space are formed inaccordance with the two weight vectors W1 and W2. By transmitting themodulated output signals riding on these beams, signals with certaincorrelations are transmitted. Then, the receiving side receives thesignals transmitted by the two beams and estimates the originaltransmission signal by maximum likelihood estimation, for example, byViterbi decoding, based on the correlation between the received signals.

In practice, the situation of the channel between the transmitting andthe reception apparatus changes all the time. In particular, in a mobilecommunication system, when the reception apparatus is, for example, amobile communication terminal, since the channel between thetransmitting apparatus and the reception apparatus changes at all times,at the transmitting side, if the beams of the transmission antenna arecontrolled in accordance with constant weight vectors, it is impossibleto deal with changes of the channel and the quality of communication maydeteriorate. Therefore, it is desirable to adapt the beams of thetransmission antenna according to the situation of the channel. One ofthe effective methods for realizing this, as in this example, is toprovide a weight vector determination circuit, set optimal weightvectors in accordance with the change of the channel, and provide thesame to the beam control circuit.

In FIG. 7, the weight vector determination circuit 150 determines theoptimum weight vectors W1 and W2 in accordance with the characteristicsof the channel and provides the same to the beam control circuit 130.The estimation of the characteristics of the channel can be realized bytransmitting a known signal between the transmitting and the receptionapparatuses. For example, before the start of the communication, thetransmission apparatus 100 transmits a predetermined pilot signal. Notethat, the pilot signal is a signal determined by a predetermined ruleand must be known by both the transmitting and the receptionapparatuses. The reception apparatus can estimate the distortioncharacteristics of the channel in accordance with the received pilotsignal. Then the characteristics of the channel estimated by thereception apparatus are transmitted to the transmitting side. The weightvector determination circuit 150 can learn the distortioncharacteristics of the channel in accordance with the estimated resultreceived from the reception apparatus and set the optimum weight vectorsaccordingly. Further, for the simplification of the reception apparatus,the reception apparatus does not estimate the channel characteristicsbut simply return the received pilot signal to the transmissionapparatus. In this case, the transmission apparatus can estimate thedistortion characteristics of the channel based on the pilot signalreturned by the reception apparatus. The weight vector determinationcircuit 150 can set the optimal weight vectors in accordance with theestimation result.

After the start of the communication, the transmission apparatus insertsthe pilot signal into the transmission signals and sends the same at apredetermined interval. Based on this, the channel characteristics canbe estimated by the transmission apparatus or the reception apparatus ata certain time interval, so that in the case of mobile communication,the optimal weight vectors can be dynamically set in accordance with thechange of the channel characteristics.

Note that the method of the determination of the weight vectors is notlimited to the examples described above. For example, when the changedpattern of the channel characteristics is known by experience, aplurality of weight vectors are calculated in accordance with thepossible values of the channel characteristics and stored in a memorydevice. During communication, the optimal weight vectors according toexperience are read from the memory device. By controlling the beamsaccordingly, although the optimal weight vectors cannot be set in realtime as described above, the weight vectors can be suitably set within acertain range of error.

Below, the reception apparatus 300 in the communication system of thepresent invention will be explained.

FIG. 8 is a view of an example of the configuration of the receptionapparatus 300. As illustrated, the reception apparatus 300 is comprisedof a reception antenna 310, a frequency conversion circuit 320, and adecoding circuit 330.

The reception antenna 310 receives the radio wave signals sent by thetransmission apparatus 100 and supplies the received signals to thefrequency conversion circuit 320.

Here, the reception antenna 310 can be comprised by a single antenna oran antenna array formed by a plurality of antenna elements. Thereception antenna is optimally controlled in its characteristics, forexample, its beam pattern, to be able to compensate for the distortionof the channel. Namely, the reception antenna can be considered as aspatial filter. The spatial filter is controlled to have characteristicsreverse to those of the channel. Accordingly, the distortion generatedin the channel is suppressed by the reception antenna, and theequalization processing of the received signals is executed in the spacedomain. A preferable example of the reception antenna 310 is an adaptivearray antenna.

The signals received by the reception antenna 310 are supplied to thefrequency conversion circuit 320 wherein the frequency is converted. Asa result of the frequency conversion, a received signal converted fromthe carrier frequency to, for example, the baseband is output. Here, forexample, the transmission apparatus transmits two transmission signalsS1 and S2 riding on different beams, while the reception antenna 310spatially separates the received signals and receives the two signalsRS1 and RS2 in accordance with the two beams of the transmitting side,respectively. The received signals RS1 and RS2 are frequency convertedby the frequency conversion circuit 320, whereby two reception datastrings RD1 and RD2 are output.

The signals transmitted by the transmission apparatus 100 havecorrelation with each other in accordance with the overlap of the beamsof the transmission antenna array 140. Namely, in the receptionapparatus 300, the two signals RS1 and RS2 received by the receptionantenna 310 have correlation with each other. Therefore, there is alsocorrelation between the two data strings RD1 and RD2 output from thefrequency conversion circuit 320.

The decoding circuit 330 estimates a decoded signal S₀ nearest to theoriginal transmitting information sent by the transmission apparatus100, that is, the transmission signal S_(i) shown in FIG. 2, inaccordance with the data strings RD1 and RD2 obtained by the frequencyconversion based on maximum likelihood estimation, for example, Viterbidecoding.

The decoding circuit constituted based on the Viterbi decoding algorithmcalculates the metric showing the distance between the codes aslikelihood functions of the path through the states of the received datastring based on the states, traces the change over time, and selectsfrom among the candidates of the different times the one having themaximum likelihood from the received signal, and finally estimates thedata string with the maximum likelihood as the original informationstring transmitted by the transmitting side. In this case, the datastring obtained is the output at the time when the path is determined inthe decoding process. The corresponding information data can be obtainedby finding the input string for this output in reverse. In this way, byutilizing the Viterbi decoding algorithm, the data string with themaximum likelihood can be found effectively by using a trellis diagram.

FIG. 9 is a trellis diagram used for decoding processing of the decodingcircuit 330 in the reception apparatus 300 shown in FIG. 8. The decodingcircuit 330 can find the estimated signal S₀ of the originaltransmission signal in accordance with the received data string usingthe trellis diagram shown in FIG. 9 based on the Viterbi decodingalgorithm.

FIG. 10 is a graph of the result of the bit error rate (BER) of thecommunication system of the present embodiment calculated by computersimulation. Here, for comparison, the bit error rate of conventionaldiversity transmission is also plotted.

Here, as a condition for computer simulation, the channel is supposed tobe in a Rayleigh fading environment. The amounts of the interference αand β between the two transmission beams B1 and B2 formed by thetransmission antenna array of the transmission apparatus are equal andα=β=0.7994. Note that, the amounts of the interference between thetransmission beams B1 and B2 are known information in the receptionapparatus.

The transmission antenna array is an antenna array constituted by sixantenna elements, while the reception antenna is an antenna arrayconstituted by six antenna elements. The transmission antenna arrayforms two transmission beams B1 and B2 and makes the angles of the mainlobes of the transmission beams B1 and B2 0 and 5 degrees, respectively.The receiving side spatially separates and receives the received signalsby the reception antenna array. Two receiving beams are formed by thereception antenna array. The angles of the main lobes of the receivingbeams are made 0 and 10 degrees, respectively.

As shown in FIG. 10, along with the increase of the signal to noiseratio SNR (for example, the ratio of the received signal power and theaverage power of noise), the bit error rate of the received signaldecreases. Under the same transmitting and receiving conditions and thesame channel environment, the bit error rate of the transmitting andreceiving method according to the present invention can be furthersuppressed in comparison with the diversity transmission scheme of theprior art. Namely, according to the communication method of the presentinvention, the error of the received signal becomes smaller and animprovement of the reliability of the communication system can berealized.

In the above embodiment of the present invention, the transmissionapparatus 100 and the reception apparatus 300 use the array antennas toform a plurality of beams and transmit and receive the signals. Thepresent invention is not limited to this. In addition to array antennas,it is possible to use other antennas capable of controlling the beampatterns. Namely, the transmission apparatus 100 can form a plurality oftransmission beams partially overlapping each other and transmit thesignals riding on the transmission beams so as to transmit the encodedsignals in both the space and time domain. The reception apparatus 300can separate and receive at least two signals having correlation witheach other by a plurality of receiving beams and estimate the originaltransmission signal by maximum likelihood estimation.

Note that the separating and reception at the receiving side are notlimited in space domain by the reception antenna beams and can also beperformed in the time or frequency domain. For example, the receivingside can separate and receive in time the two transmission signalstransmitted shifted in time by the transmission apparatus 100 a shown inFIG. 6 so as to receive two signals having correlation with each other.Further, when the transmitting side transmits the two signals modulatedto different carrier frequencies by different transmission beams, thereceiving side can separate and receive the two transmission signalsaccording to the difference of the frequencies of the signals.

As described above, according to the transmission apparatus and thecommunication system of the present invention, the transmitting sidetransmits the transmission signals encoded by different encodingprocessing riding on a plurality of transmission beams by using atransmission antenna forming a plurality of beams partially overlappingeach other and the receiving side separates and receives signals anddecodes the original transmission signal by maximum likelihoodestimation considering the correlation between the received signals soas reduce the error rates of the decoded signals and realize animprovement of the efficiency of the communication system andimprovement of the quality of communication.

Furthermore, by Viterbi decoding processing as the maximum likelihoodestimation in the reception apparatus, there are the advantages thatreal time decoding can be realized and simplification of the receptionapparatus and the higher speed of the decoding process can be realized.

1. A transmission apparatus comprising: an encoding means for encoding atransmission signal to generate at least a first transmission signal anda second transmission signal, the encoding means including a delaycircuit for delaying the second transmission signal by a predetermineddelay time compared to the first transmission signal, a transmissionantenna forming at least a first beam and a second beam, and atransmitting means for transmitting said first transmission signalriding on said first beam and transmitting said second transmissionsignal riding on said second beam, wherein said first beam and secondbeam are formed so as to partially overlap each other.
 2. A transmissionapparatus as set forth in claim 1, wherein the transmission antenna isan array antenna comprising a plurality of antenna elements.
 3. Atransmission apparatus as set forth in claim 2, wherein the transmittingmeans weights the first transmission signal and the second transmissionsignal modulated to predetermined carrier frequencies with predeterminedweights and supplies them to the antenna elements.
 4. A transmissionapparatus as set forth in claim 3, further comprising a weightdetermining means for determining weights of the antenna elements andcontrolling the beam patterns of the first beam and the second beam. 5.A transmission apparatus as set forth in claim 4, wherein the weightdetermining means determines the weights of the antenna elements inaccordance with channel characteristics.
 6. A transmission methodcomprising: a step of encoding a transmission signal to generate atleast a first transmission signal and a second transmission signal, thestep including delaying the second transmission signal by apredetermined delay time compared to the first transmission signal, astep of modulating the first transmission signal and the secondtransmission signal to predetermined carrier frequencies, a step ofweighting the first transmission signal by a first weight, supplying thesame to antenna elements constituting a transmission antenna,transmitting the same by forming a first transmission beam and ofweighting the second transmission signal by a second weight, supplyingthe same to antenna elements, and transmitting the same by forming asecond transmission beam partially overlapping the first transmissionbeam.
 7. A transmission method as set forth in claim 6, furthercomprising determining the first weight and the second weight inaccordance with channel characteristics.
 8. A communication systemcomprising: an encoding means for encoding a transmission signal togenerate at least a first transmission signal and a second transmissionsignal, the encoding means including a delay circuit for delaying thesecond transmission signal by a predetermined delay time compared to thefirst transmission signal, a transmission antenna forming at least afirst transmission beam and a second transmission beam, a transmittingmeans for transmitting said first transmission signal riding on saidfirst transmission beam and transmitting said second transmission signalriding on said second beam, a reception antenna forming a predeterminedreceiving beam and receiving signals transmitted by said transmissionantenna using the receiving beam, and a decoding means for estimatingthe transmission signal by a maximum likelihood estimation according tothe received signals of the reception antenna, wherein said firsttransmission beam and second transmission beam are formed so as topartially overlap each other.
 9. A communication system as set forth inclaim 8, wherein the transmission antenna is an array antenna comprisinga plurality of antenna elements.
 10. A communication system as set forthin claim 9, wherein the transmitting means weights the firsttransmission signal and the second transmission signal modulated topredetermined carrier frequencies with predetermined weights andsupplies the same to the antenna elements.
 11. A communication system asset forth in claim 10, further comprising a weight determining means fordetermining weights of the antenna elements and controlling the beampatterns of the first beam and the second beam.
 12. A communicationsystem as set forth in claim 11, wherein the weight determining meansdetermines the weights of the antenna elements in accordance withchannel characteristics.
 13. A communication system as set forth inclaim 8, wherein the reception antenna is an array antenna comprising aplurality of antenna elements.
 14. A communication system as set forthin claim 8, wherein the reception antenna shapes the beam pattern of thereceiving beam in accordance with the transmission distortion of achannel.
 15. A communication system as set forth in claim 8, furthercomprising a separating receiving means for separating the receivedsignal by the reception antenna.
 16. A communication system as set forthin claim 15, wherein the decoding means estimates the originaltransmission signal based on a correlation between at least two receivedsignals output from the separating receiving means.
 17. A communicationsystem as set forth in claim 8, wherein the reception antenna forms afirst receiving beam and a second receiving beam and separates andreceives the signals transmitted by the transmission antenna by thefirst receiving beam and the second receiving beam.
 18. A communicationsystem as set forth in claim 17, wherein the decoding means estimatesthe original transmission signal based on a correlation between thereceived signal received by the first receiving beam and the receivedsignal received by the second receiving beam.
 19. A communication systemas set forth in claim 17, wherein the decoding means estimates theoriginal transmission signal based on a correlation between the receivedsignal received by the first receiving beam and the received signalreceived by the second receiving beam using a Viterbi decodingalgorithm.
 20. A communication method comprising: a step of encoding atransmission signal to generate at least a first transmission signal anda second transmission signal, the step including delaying the secondtransmission signal by a predetermined delay time compared to the firsttransmission signal, a step of modulating the first transmission signaland the second transmission signal to predetermined carrier frequencies,a step of weighting the first transmission signal by a first weight,supplying the same to antenna elements constituting a transmissionantenna, and transmitting the same by forming a first transmission beamand of weighting the second transmission signal by a second weight,supplying the same to antenna elements, and transmitting the same byforming a second transmission beam so as to partially overlap the firsttransmission beam, a step of shaping a beam for compensation of thetransmission distortion of a channel and receiving the signals sent bythe transmission antenna by the shaped beam, and a step of estimatingthe original transmission signal in accordance with the received signalsby the reception antenna by maximum likelihood estimation.
 21. Acommunication method as set forth in claim 20, further comprisingdetermining the first weight and the second weight in accordance withchannel characteristics.
 22. A communication method as set forth inclaim 20, further comprising a step of forming a first receiving beamand a second receiving beam and separating and receiving the signalstransmitted by the transmission antenna by the first receiving beam andthe second receiving beam.
 23. A communication method as set forth inclaim 22, further comprising a step of estimating the originaltransmission signal by maximum likelihood estimation based on acorrelation between the received signal received by the first receivingbeam and the received signal received by the second receiving beam. 24.A communication method as set forth in claim 22, further comprising astep of estimating the original transmission signal based on acorrelation between the received signal received by the first receivingbeam and the received signal received by the second receiving beam usinga Viterbi decoding algorithm.